This invention relates generally to pneumatic pressure controllers and in particular, to a differential pressure controller for regulating the velocity of air flowing through a duct.
As is known, heating, ventilating and air conditioning (HVAC) systems are used to control, among other parameters, the temperature of a space to be conditioned. An example of such a space may be one or more rooms in an office building. In earlier systems, the manner of controlling temperature was to provide a thermostat within the space for establishing the setpoint or temperature to be maintained therein. If cooling was required to maintain this setpoint, as in summertime, the volume of air flowing through a duct and into the space was maintained at a constant value while the temperature of that air was regulated to provide the desired level of cooling. Since the duct air flow rate or velocity was essentially constant, the volume of air flowing therethrough was likewise substantially constant, the velocity and volume having a known relationship for a duct of a known cross-sectional area. These earlier systems were characterized by at least two disadvantages.
First, the velocity of air flowing through the duct tended to vary due, in part, to changes in the duct static pressure caused by adjustments in flow within other duct systems within the building. Second, the temperature of this air was frequently controlled by mixing warmer and cooler air and the energy consumed by that process was undesirably high and therefore wasteful.
In more recent HVAC systems, a variable air volume (VAV) scheme has been adapted. In a scheme of this type and if, for example, space heating was required as in a wintertime application, the temperature of the air flowing through the duct is maintained at a relatively constant value, within a range of, for example, 20.degree.-40.degree. above thermostat setpoint. The volume of air flowing through the duct is thereupon increased, maintained or decreased for maintaining the thermostat setpoint temperature, even though the space heating load may vary. Similarly, if cooling is required to maintain a temperature within a space, the duct air temperature will be maintained at a relatively constant value within a range of, for example, 15.degree.-20.degree. F. below the thermostat setpoint. Thermostats typically used with VAV systems are of the pneumatic type and provide a pressure output signal useful for control purposes.
Differential pressure controllers as embodied by the instant invention and as disclosed in U.S. Letters Patent discussed below therefore employ a parameter known as the velocity or kinetic pressure to effect control. Since velocity pressure is not readily, directly obtained within an air duct, it may be determined by sensing (a) the velocity+static pressure, sometimes termed the stagnation pressure, on one hand and (b) the lower static pressure on the other hand and subtracting the static pressure from the stagnation pressure to obtain the velocity pressure component. In the invention and in other known differential pressure controllers, the subtraction of these pressures is effectively resolved as a differential pressure across a sensing diaphragm.
One known way of obtaining the stagnation pressure component is by the disposition of a Pitot tube within the duct with the Pitot nozzle directed upstream parallel to the axis of air flow. The static pressure may be obtained by a static pressure tube disposed within the duct and having its axis oriented normal to or downstream of the air flow. Having thus obtained the velocity pressure, the air velocity may be computed using the known formula ##EQU1## where V=velocity in ft./min., Cp=the velocity coefficient, Pv=velocity pressure in inches water gauge, g=32.2 ft/sec.sup.2 gravity and da=the air density in lbs./cu. ft. Thereafter, the volumetric rate of air flow through the duct may then be readily determined by the known formula Q=V.times.A where Q=air quantity in cu.ft./min. and A=the cross-sectional area of the duct in sq. ft.
The aforedescribed approach to VAV system control will be satisfactory so long as the static pressure in the duct remains unchanged. In practice, this static pressure is likely to change due to air flow protuberances as flow controlling dampers in other parts of the HVAC system are opened or closed. Therefore, a more desirable differential pressure control for use in VAV systems will be capable of being interlocked to and resettable or readjustable by the room thermostat. The control will thereby be capable of maintaining a constantly regulated volume of air into a space where the volume flow is in proportion to the requirement set by the thermostat and independent of variations of static pressure within the duct. It is also desirable that such a controller be adaptable to limit the maximum and minimum velocities of air flowing through the duct and is also capable of exhibiting satisfactory control characteristics near zero velocity settings. This latter function is made more difficult by the fact that, near zero velocity settings, small changes in duct static pressure and the resulting shift or offset in the setpoint of the controller will affect the velocity of air in the duct to a greater extent than would occur with the same change in static pressure at higher velocity settings.
Several considerations impact upon the design of a preferred differential pressure controller, one of them being related to the fact that such controllers may be used in any one of four, commonly encountered control modes, namely direct acting with direct or reverse readjustment and reverse acting with direct or reverse readjustment. These modes are described in further detail following. Therefore, a preferred controller will be readily connectable for use in any one of the four control modes without modification and without the necessity of employing a device commonly known as a reversing relay. A controller designed in this preferred manner may be manufactured and stocked by sellers in only a single configuration and, further, the sellers' inventory may be reduced if reverse relays are no longer required.
Yet another consideration relates to the fact that such controllers typically operate on extremely low pressures, typically on the order of under one inch water column, one inch water column pressure being approximately equivalent to 1/27th p.s.i.g. Therefore, it is important that such a controller be configured to eliminate or at least minimize friction between sliding components and be further configured to eliminate changes in performance characteristics due to wearing of the parts.
One approach to the design of such differential pressure controllers is shown in U.S. Pat. No. 4,077,567. Since the device shown therein provides for the application of a thermostat control signal component to a separate reset diaphragm and pressure chamber rather than to one of the main pressure chambers, it is useful only in the direct acting, direct readjustment mode. Additionally, the device shown therein employs a pinned, pivotable arm, the progressive wearing of which may result in loss of accuracy. It further employs a reset pin coacting with a leaf spring, the interface of which may result in unnecessary sliding friction as will the use of an arbor and a piston, both being slidable within bores. Further, that device is unnecessarily complex in its use of multiple piece parts.
Yet another apparatus for controlling the volume of flow within a duct is shown in U.S. Pat. No. 3,941,310. This apparatus is similar to the aforementioned in its use of a pivoted lever. Additionally, this apparatus uses only one sealing diaphragm and a device so constructed may be subject to undesirable drift with changes in duct static pressure because of unequal effective (net) areas at either side of the main sensing diaphragm.
A differential pressure controller which resolves velocity pressure and employs a pair of chambers, one of which senses a thermostat control signal component, which employs a coaxial force balance design to be readily reconnectable to any one of four control modes, which is made substantially frictionless by avoiding pivoted levers and pistons sliding within bores, which may be configured to eliminate controller offset otherwise resulting from changes in static pressure and which may be readily adapted to maintain the readjustment signal between preselected high and low limits would be a distinct advance in the art.